(1) Field of the Invention
This invention relates to conduits, and more particularly to oil field tubing in wells. Applicant submits that an oil field production person is one having ordinary skill in this art.
(2) Description of the Related Art
It is well known that tubing is used in wells in oil field production. A common example is that tubing which goes inside of a casing and has a pump attached to the bottom in an oil bearing formation. A sucker rod extends from the pump in an oil bearing formation to the top of the tube.
Another example of oil field tubing includes tubing with a submergible motor and pump attached to the bottom. The oil is produced by pumping the oil from the oil bearing formation far below the surface of the ground to the surface through the open tubing.
Other uses include tubing in a water flood field where tubing is used in injection wells and water is pumped down the tubing to an oil bearing formation far below the ground to displace the oil to production wells in the field.
Normally, the string of tubing in an oil well will include individual sections of tubing, e.g. 20' or 30' long having external threads on each end of each section. The sections are connected together by cuffs having internal threads. Because the external threads on the ends of the section would otherwise weaken the tubing, it is common practice to use tubing having upset ends. The ends of the tubing are thicker so that after the threads are cut in them, they still have the necessary tensile strength to support long strings of tubing. It is not unusual to have 10,000' of tubing which is supported from the surface of the earth. Therefore, the tubing must have sufficient tensile strength at the surface to support the weight of 10,000' of tubing.
Also, in many instances, other devices besides sucker rods are passed through the tubing. Although normally these are lowered by a wire line, in certain instances, these devices may be dropped for free fall through the tubing. Therefore, it is extremely important that the tubing have uniform bore from top to bottom, and it is important that the tubing not have any restrictions in the bore from top to bottom, and it is desirable that the tubing not have abrupt changes or ledges in its bore upon which devices could hang. Also, it will be recognized that smooth bore has less resistance to the flow of fluid through it than a bore having abrupt changes. Therefore, it is desirable that the bore be smooth, or if there are any changes in diameter that there be a smooth transition between any changes in diameter.
In many instances the fluids flowing through the tubing will be highly corrosive. Therefore it is known in the art to place a corrosive resistant coating on the inside of the tubing to prevent the fluid passing through the tubing from corroding it.
The joints at each coupling form a corrosion problem. Under normal circumstances, the threaded joint between the tubing and coupling forms a fluid tight seal for joining one tubing section to the next. Because of the mechanical nature of screwing one section to the next, it is difficult, if not impossible, to form a corrosion resistant coating at the threads.
Before this application was filed, the applicant was aware of the following United States patents:
______________________________________ Inventor Issue Number ______________________________________ Hinderliter Re. 19,690 Doolittle 262,581 Levier 354,560 Havens 1,925,533 Douglass 2,349,081 Eirhart 3,163,450 Miller 3,286,341 Krieg 3,427,050 Gerner 3,461,918 Grahl 3,722,923 Dula et al. 3,811,710 Gold 3,866,958 Gottlieb 4,026,583 Lula 4,366,971 Fruck 4,400,019 Werner 4,507,842 Yoshida et al. 4,509,776 Axford et al. 4,568,113 Handa et al. 4,623,173 ______________________________________
Many of these patents represent efforts of prior workers to solve this problem of the corrosion at the cuffs. Other of these patents show structure similar to this invention; although, the invention was directed to the solution of some other problem.